As is well known by those skilled in the art, a microstrip is comprised of a planar substrate of dielectric material having a conductive layer covering one side that serves as a ground plane and a top conductor that is about one-tenth the width of the substrate extending along the center of its other side. The top conductor and the ground plane form a waveguide through which microwaves can pass in either direction. When a resonance isolator is incorporated with the microstrip, it absorbs energy from microwaves flowing in one direction so as to block them, but does not absorb energy from microwaves flowing in the other direction so as to permit them to pass.
In one form of resonance isolator, the top conductor is formed in a circular path and microwaves are permitted to flow in only one direction around the circle, but this takes up considerable space on the surface of the substrate.
In U.S. Pat. No. 4,806,886, which is incorporated herein by reference, a resonant isolator is described that lies along the top conductor. A gap is formed in the top conductor during the etching process, and a waveguide element having a dielectric constant significantly greater than that of the substrate is mounted within the gap. The waveguide element has a central section of uniform height and a ramp at either end, and a conductive strip is adhered to its top and joined by soldering to the top conductor at the ends of the gap. This structure requires a number of manufacturing steps that greatly increase the cost.